The present invention relates to an improved ferrule used to form an optical connector for optical fibers. The ferrule is used to connect optical fibers by joining pairs thereof in a holding sleeve. Mechanical properties of optical connector ferrules greatly affect the connecting characteristics of optical fibers.
Optical connector ferrules using a metal sleeve on the outer periphery thereof have previously been proposed in the art. An example of such an optical connector ferrule in which a metal sleeve is provided around the outer periphery of one portion thereof is disclosed in Japanese Published Unexamined Patent Applications Nos. 42207/81 and 27112/83. A modification of such a structure in which the metal sleeve is prevented from separating from the synthetic resin material forming the body of the ferrule is disclosed in Japanese Utility Model Publication No. 38487/81. In the optical ferrule of the Utility Model Publication, radial holes are formed in the metal sleeve into which flows the synthetic resin material at the time of molding.
FIG. 1 is a cross-sectional schematic illustration of an optical connector ferrule as disclosed in the above-mentioned Japanese Published Unexamined Patent Applications. In this ferrule, a body 3 of synthetic resin material is formed with a radially extending flange 31. A coated optical fiber 1 extends through most of the length of a central longitudinal bore in the body 3. A forward end 2 of the optical fiber 1 is stripped of its protective coating. A metal sleeve 4 is disposed around the forward end of the body 3, extending from the forward side of the flange 31 to the forward end of the body 3.
Epoxy resins typically used to form the body 3 of the optical ferrule have a shrinkage upon curing at about 0.490 to 0.670 percent. This degree of shrinkage is larger than the coefficient of thermal expansion of the metals typically used to form the metal sleeve 3. Accordingly, the resin and sleeve have a tendancy to separate following curing. If the inner diameter of the metal sleeve 4 is, for instance, 2 mm, the resin may shrink by about 10 microns, producing an equal amount of eccentricity in the position of the optical fiber. This results in a large amount of loss when a connection is made.
In order to prevent separation of the metal sleeve from the main body of the ferrule, as proposed in the above-mentioned Japanese Utility Model Publication, radial holes may be formed in the metal sleeve so that the resin flows therein during molding, thereby tightly securing the metal sleeve to the body of resin material. However, providing such holes in the metal sleeve requires extra processing, significantly increasing the overall cost of the ferrule. Moreover, the resin flowing through the holes in the metal sleeve changes the overall outer diameter of the ferrule, which is disadvantageous in achieving accurate alignment between connected optical fibers.
Another important problem with the conventional optical ferrule is a lack of bending strength. In the conventional optical connector ferrule as shown in FIG. 1 wherein the metal sleeve extends from the side of the flange to the forward end of the resin body, the maximum bending strength is about 2 kg. It is desirable that the bending strength of the connector be at least twice that value.
Another drawback of the conventional optical connector ferrule arises during the polishing of the end surface of the bared optical fiber 1. To polish the end of the optical fiber 1, it is necessary to polish the resin material and metal simultaneously, which is quite difficult to do because of the different hardnesses among the three materials.